It has been demonstrated that the T cell proliferative response to Con A and the T cell dependent IgM responses to trinitrophenyl (TNP) conjugated keyhole limpet hemocyanin (KLH) require the participation of an accessory population. This subpopulation of spleen cell is glass adherent, non-T, non-B, 1500 R resistant and expresses Ia (I-region associated) determinants encoded by genes in I-A and I-E/C and these determinants are expressed on the same cell. Further characterization of this accessory population revealed that for the antibody response the accessory cell appears to be phagocytic and H-2 identity was required with the T cells. However, for the Con A response the cell appears to be non-phagocytic and does not have to be H-2 identical with the responding T cells in order to perform its accessory function.